This invention relates to a droplet generator for a continuous stream ink jet print head.
More particularly the invention relates to such a generator comprising: an elongate cavity for containing the ink; nozzle orifices in a wall of the cavity for passing ink from the cavity to form jets, the nozzle orifices extending along the length of the cavity; and actuator means disposed on the opposite side of said cavity to said wall for vibrating the ink in the cavity by itself vibrating relative to the wall, the vibration being such that each jet breaks up into ink droplets at the same predetermined distance from the wall of the cavity. Droplet generators of the aforegoing type will hereinafter be referred to as droplet generators of the specified type.
In order to enable generators of the specified type to be used with corrosive (non-aqueous based) ink, certain known such generators are constructed predominantly of stainless steel components. One such component is the wall containing the nozzle orifices, and takes the form of a thin sheet of stainless steel foil through which the orifices extend.
The orifices have to be comparatively small and of very high quality. This is so that the jets produced by the orifices are identical. They must be parallel to one another to fractions of a degree, and have equivalent velocities to within a few percent. This requires perfectly round holes with relative sizes to within 5 percent. There are few fabrication techniques that can achieve this requirement in stainless steel. All techniques suffer and encounter increasing difficulty as the thickness of the foil increases. The superior technique evolved is electro discharge machining (EDM).
In such an orifice formation process a thin metal wire or electrode is brought to within close proximity of the foil. A voltage is applied across the gap and as arcing occurs between the foil workpiece and the electrode local heating results in vaporisation and expulsion of the foil material. In order to achieve holes of the required quality very low current is applied. This improves the finish of the holes but increases the time required to xe2x80x98drillxe2x80x99 each hole, and hence complete the drilling of the full array of holes/orifices. In a 128 dots per inch (DPI) printer having a 50 mm long line of 256 holes, each extending through foil 100 xcexcm thick, the drilling time amounts to 12-13 hours. This time is considerable and has significant production implications with respect to both unit cost and capacity.
The measure of an ink jet printer""s ability to print on distant substrates is termed the xe2x80x98throwxe2x80x99 of the printer. A high throw is necessary when printing on uneven substrates or in conditions where there is significant air turbulence in the region of the jets. Throw is related to jet velocity. Jet velocity equals wavelength multiplied by frequency. Vibration of the actuator means at the frequency of operation of the generator produces an ultrasonic wave which travels down the jets. This wave is clearly visible in the jets under suitable magnification, and enables wavelength and therefore jet velocity to be measured. For a given frequency of operation, wavelength can be used as a measure of jet velocity and hence throw of a printer. It can be seen that at a given frequency of operation it is desirable to maximise jet wavelength to maximise throw.
In a known ink jet printer, having a standard 128 DPI nozzle produced in 100 xcexcm thick stainless steel foil, when using methylethylketone ink, the operating range of wavelengths is 155 to 165 xcexcm, giving a mean operating wavelength of 160 xcexcm representing a jet velocity of 12 m/s.
According to the present invention there is provided a droplet generator for a continuous stream ink jet print head comprising: an elongate cavity for containing the ink; nozzle orifices in a wall of said cavity for passing ink from the cavity to form jets, said nozzle orifices extending along the length of said cavity; and actuator means disposed on the opposite side of said cavity to said wall for vibrating the ink in said cavity by itself vibrating relative to said wall, the vibration being such that each said jet breaks up into ink droplets at the same predetermined distance from said wall of the cavity, the thickness of said wall through which said nozzle orifices extend being less than 90 xcexcm, said wall comprising a planar member secured to the remainder of said droplet generator so as to form a boundary which extends around said nozzle orifices and within which said planar member is unsupported, said boundary including first and second boundary lengths which extend along the length of said cavity on either side of the nozzle orifices, the distance between said first and second boundary lengths being less than 1700 xcexcm.
Preferably, the distance between said first and second boundary lengths is less than 1350 xcexcm.
Preferably, said planar member is a planar metallic member, e.g. stainless steel foil. Preferably, said planar metallic member is secured to the remainder of said droplet generator by means of welding, the path taken by the welding defining said boundary around the nozzle orifices. Preferably, the nozzle orifices have been formed in said planar metallic member by electro discharge machining.
Preferably, said thickness of said wall through which said nozzle orifices extend is greater than 45 xcexcm, more preferably greater than 55 xcexcm, even more preferably from 60 to 80 xcexcm.